In recent years, the demand for SMC and BMC prepared by using unsaturated polyester resins as the base material have steadily been increasing in the processing industry of reinforced plastics as novel industrial materials for machine molding satisfying the requirements of labor saving, mass production, and improvements in thw working environment.
SMC stands generally for a sheet material prepared as follows: an oxide or a hydroxide of an alkaline earth metal used as a thickening agent (an agent for preparation of a sheet) is added to a fluid dispersion obtained by mixing an unsaturated polyester resin, a liquid polymerizable monomer, a curing agent, an inorganic filler, a mold release agent, a flame retardant, a pigment, and the like; a fluid composition is prepared by adding a reinforcing material, such as glass fiber, to the obtained fluid dispersion in an amount of 20 to 50 parts by weight per 100 parts by weight of the fluid dispersion in order to provide the fluid composition with toughness; the prepared fluid composition is sandwiched between release films; thickening of the fluid composition is allowed to take place by reaction for several days; and the resultant fluid composition is formed into a sheet.
The sheet prepared in the above is transferred to the place for molding and molded by heating in a mold to obtain a molded product having a finished shape. The molding is generally conducted by pressing the sheet under a condition of a high temperature and a high pressure, such as a temperature of 100 to 180.degree. C. and a pressure of 30 to 100 kgf/cm.sup.2 (gauge).
Because shrinkage of about 7 to 10% by volume takes place with the molded product obtained by the pressing of the sheet, addition of a thermoplastic resin as a low shrinkage agent is generally conducted for improving the shrinkage (Japanese Patent Publication Showa 46(1971)-14541, Japanese Patent Application Laid-Open No. Heisei 6(1994)-32809, and like others).
Although the technology for molding has been improved by the above process, and unsaturated polyester resins have increasingly been used, unsaturated polyester resins have not widely been used for articles produced in small numbers because molds which are durable under a high pressure are expensive, and other facilities for high temperatures and high pressures are necessary.
Thus, development of an unsaturated polyester resin composition for SMC which is easily processed in molding at a low temperature (40 to 100.degree. C.) and/or under a low pressure (0.1 to 10 kgf/cm.sup.2), even under a vacuum, has been desired.
In particular, when the molding is conducted at a low temperature, a curing agent of a low temperature decomposition type must be adopted. When the molding is conducted under a low pressure, the molding must be conducted while the viscosity of SMC during heating is decreased by adjusting the formulation of the compound, for example, by decreasing the added amount of a metal oxide or an inorganic hydroxide which is mixed as a filler or as a flame retardant or by decreasing the mixed amount of glass fiber which is used as a reinforcing material.
However, when the SMC described above which can be molded at a low temperature is prepared, it is necessary that a curing agent having a low decomposition temperature be added, and this curing agent tends to have a shorter period for service at a room temperature because reaction takes place with an oxide of an alkaline earth metal or the like which is used for increasing the viscosity of the composition to facilitate the preparation of the SMC. Therefore, it is necessary that procedures including the press molding be conducted within a short time or the SMC be stored at a low temperature. When the amount of an oxide of an alkaline earth metal or the like is decreased in order to solve the above problem, various problems in the low pressure molding arise as described in the following.
When the low pressure molding is conducted, it is necessary that the added amount of an alkaline earth metal or the like be decreased in order to facilitate flow of the unsaturated polyester resin composition. However, the decrease in the above amount causes insufficient increase in the viscosity of the SMC itself, and cleavage of the release film used for sandwiching the SMC sheet becomes unsatisfactory. This requires a complicated operation in which the SMC sheet is molded while the release sheets are still attached thereto and the release films are removed from the molded product after the molding. Moreover, because the molding is conducted while the release sheets are still attached to the SMC sheet, molding of a thick article by using a laminate of a plurality of SMC sheets is difficult. As still another problem, when an oxide of an alkaline earth metal is present even in a small amount, the reaction with a curing agent continues to proceed slowly at a room temperature to cause decrease in the storage stability of SMC, and the period for service is inevitably limited. As still another problem, when the amount of glass fiber added to the compound is decreased in order to adjust the viscosity, strength of the obtained molded product is decreased.
In the conventional method for increasing the viscosity by using an oxide of an alkaline earth metal as the thickening agent, the thickening agent and the unsaturated polyester resin are brought into a crosslinking reaction while decomposition of the curing agent is avoided. Therefore, a treatment at a room temperature or a treatment by heating at a low temperature is required, and it generally takes several days for increasing the viscosity to a prescribed value. Thus, development of a process for preparation of SMC in a short time has been desired.
Under the circumstances described above, the present invention has an object of providing an unsaturated polyester resin composition which enables preparation of SMC in a short time and provides SMC having an excellent storage stability, showing no problem on cleavage of release films, and enabling molding at a low temperature under a low pressure and another object of providing a process for molding the composition.